CARACAS, Venezuela — From mangrove swamps in Venezuela to lowland forests in Indonesia, entire communities of plants and animals are under threat. Now scientists are figuring out how to catalog and map the world's most threatened ecosystems — just like their familiar list of endangered species.

Some experts say drawing up a global "Red List" of vanishing ecosystems would help them spot looming crises caused by everything from climate change to the cutting of forests, and would sharpen their focus on areas to conserve.

Along the shore of Venezuela's Lake Maracaibo, runoff filled with sediment and pesticides has been smothering animals that once lived among the roots of the mangrove trees, including crabs, fish hatchlings and shellfish, said Luz Esther Sanchez, a marine biologist and ecologist. She said saving the mangroves requires a comprehensive effort to reduce water pollution and halt the clearing of other forests upstream.

"Declaring the mangrove ecosystem threatened would be very useful for conservation," Sanchez said. "People stand up to defend dolphins. People stand up to defend turtles. But I've never seen them defend the mangrove forest with the same vehemence."

An international working group of biologists and conservation experts has been developing a system for classifying threats to ecosystems.

"If we can get a good, rigorous scientific system in place that is relatively easy to monitor worldwide ... you can follow these changes and describe them and ring the alarm bell where things might go wrong," said Dutch conservation expert Piet Wit.

He chairs the Commission of Ecosystem Management of the International Union for Conservation of Nature, or IUCN, which maintains the Red List of thousands of threatened plants and animals worldwide.

Some scientists caution that agreeing on precise categories to divvy up habitats would be a monumental task. But many already agree on some ecosystems that are threatened or endangered, including many coral reefs, salt marshes, mountain habitats threatened by rising global temperatures, grasslands in southern Russia and Brazil's Atlantic forest.

Logging poses a serious threat to the lowland forests on Indonesia's Borneo Island that are home to endangered orangutans. In the Andes, expanding farmland has fragmented the cloud forests where spectacled bears live.

"You usually get ecosystem decline occurring first, and then species decline later on," said Jon Paul Rodriguez, a conservation biologist at the Venezuelan Institute of Scientific Research who is leading the IUCN working group. He and 20 other experts laid out their proposals in an article published online by the journal Conservation Biology in November.

The list of habitats devastated by people has been growing. The once-vast Aral Sea between the former Soviet republics of Kazakhstan and Uzbekistan shrank by about 90 percent due to diversions of water, leaving behind a salty wasteland and abandoned fishing boats, and ruining the local economy. North American tall grass prairies have largely vanished, along with the game animals that once thrived in them. Some rivers, such as the Rio Grande, have been strangled by heavy pumping and now barely reach the sea.

Today, some efforts to save threatened species appear to be working. Humpback whales, for instance, have rebounded from "vulnerable" to being at low risk of extinction due to the international ban on commercial whaling. Strict regulations have helped the recovery of some fish that were once heavily overfished, such as striped bass along the U.S. mid-Atlantic coast.

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One study released in October by a large international team of researchers found that efforts to save endangered animals are making a difference for dozens of species. The report concluded that the overall march toward extinction would have been about 20 percent faster if no conservation steps had been taken.

"Species Red Lists have already been a huge policy success, so there's reason to think that ecosystem Red Lists could be too, and could complement them," said Kathryn Rodriguez-Clark, an ecology and conservation specialist at the Venezuelan Institute of Scientific Research who is part of the IUCN effort.

Scientists and environmentalists have previously come up with their own systems for targeting habitats in danger of disappearing. Researchers in South Africa, Australia and Venezuela have begun to list and map out threatened ecosystems.

But the new effort aims to come up with a uniform, standard system of classifying habitats. Rodriguez said such a system could help identify new conservation goals, drawing on data from satellite images.

Stuart Pimm, a Duke University professor and expert on preventing species extinction, called the effort a good idea but said he is skeptical about how it can be put into practice.

"It's much, much more difficult to define an ecosystem than it is to define a species," Pimm said. "And the more finely you define things, the more tricky it's going to be."

Pimm said he is concerned it would be difficult to come up with "a consistent set of definitions that will survive political pressure" and hold up when tough environmental management decisions need to be made.

In the case of the mangroves that line Lake Maracaibo near the Venezuelan coast, Sanchez said species-focused conservation efforts alone won't work because the habitat is being degraded by muddy water from areas where mountain forests have been felled. She said runoff filled with pesticides is also likely taking a toll, and that in some parts of western Venezuela — which is dotted with oil rigs — leaks from oil pipelines have done damage.

"If the watershed isn't protected ... it's inevitably going to die, which is what's happening," she said. "It's all connected."

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